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Scientists believe they have a plausible explanation for Italian toads' apparent ability to sense the imminent earthquake that struck the city of L'Aquila on 6 April 2009.

By chance, Dr Rachel Grant of the UK's Open University was monitoring a toad population at San Ruffino Lake, around 74km from the quake's epicentre. As we previously reported, five days before the 6.3-magnitude shake, "the number of male common toads in the breeding colony fell by 96 per cent", while "most breeding pairs and males fled" three days before the earth moved.

At the time, Grant speculated that this unusual behaviour - when the bufo bufo population at the lake should have been busy spawning - may have reflected the toads' instinct to escape to higher ground to avoid croaking it, "possibly where they would be at less risk from rock falls, landslides and flooding".

As to what stimulus caused the exodus, Grant's initial findings suggested that toads "are able to detect pre-seismic cues such as the release of gases and charged particles, and use these as a form of earthquake early warning system".

Cue further investigation in collaboration with a NASA team, which has indeed fingered charged particles as a likely cause of the toads unexpectedly hopping off.

The NASA scientists, led by geophysicist Dr Friedemann Freund, demonstrated that rocks under extreme tectonic stress release charged particles, which travel through the surrounding rock and into the atmosphere. Their electrical charge is sufficient to ionize the air, creating "massive amounts" of positive airborne ions.

The researchers cite "unusual pre-earthquake conditions in the ionosphere" around L’Aquila as evidence of such activity, which creates an unfriendly atmosphere for animal life.

Dr Freund explained to the BBC: "Positive airborne ions are known in the medical community to cause headaches and nausea in humans and to increase the level of serotonin, a stress hormone, in the blood of animals."

Furthermore, the ions might have reacted with the toads' pond water, brewing up potentially-toxic hydrogen peroxide.

The scientists conclude that there's "little doubt that anomalous animal behaviour does occur prior to major earthquakes", and that "given the variety of physical and chemical processes documented ... it would in fact be surprising if animals were not affected".

They note that their proposed "possible common mechanism" of pre-earthquake physical phenomena and animal behaviour requires further investigation to determine its future application in quake prediction.

The full findings are published in the International Journal of Environmental Research and Public Health, right here [PDF]. ®